This intercom have 3 amplification stages?

[ae4jm]

Thanks to Audioguru and kChriste, I'm understand the circuit fairly well now on the flip-flop. This circuit is for an intercom system. I understand most of the circuit, I think, up until the point where the PNP transistor is being used. This intercom was a GE and was labeled as a EF-130. I can't figure out if there are two stages of amplification, then what would the PNP configuration to the far right be used for? Is this a 3rd stage amplifier, or is two stages used for amplification and one stage to control the modulation from the microphone frequency? One more question and I'll leave the experts alone to help others out--when one spkr is used as a mic and the other as a speaker, I suppose that the transistors just act in reverse order when the speakers are switched for input and output?

I've attached a crude schematic, I messed up on the variable resistor and there was a switch that switched both speakers to left side as input and right side as output and vice verse.

Basically, I'm trying to figure out what is happening in the circuit when each of the speakers are used as an input and output.

Thanks,
Matt

 

[audioguru]

The circuit is a 3 transistor amplifier.
The left transistor is the input amplifier, the middle transistor is the middle amplifier and the right PNP transistor is the output amplifier.
The switch swaps the speakers.

What is "control the modulation from the microphone frequency"? Your voice creates the sound, one speaker picks it up as a microphone and the transistors amplify it and drive the ouput speaker.

 

[ae4jm]

Thanks Audioguru,

I have a few more questions if you or anyone else could help clear a few things up.

So the NPN transistors are forward biased and the PNP is reverse biased any time that the microphone is feeding audio to the transistors?

C-2 is a bypass capacitor? C1, C3, and C4 are coupling capacitors? C5 is a filter capacitor? Then the resistors act as the load resistors, current limiters, and current dividers?

I'm not all that familiar with a lot of circuits and configurations, but how would a DPDT switch work to switch the speakers around so that if the left side speaker is the mic and the right-hand speaker is used as the output speaker and then the DPDT switch is thrown and the speakers switch so that now the left-hand side speaker is the output and the right-hand side speaker is the input.

Thanks for all of the excellent insight and for helping with the functioning of this circuit.

 

[audioguru]

So the NPN transistors are forward biased and the PNP is reverse biased any time that the microphone is feeding audio to the transistors?

No. The transistors are not forward biased nor are they reverse biased. They are properly biased as amplifiers.

C-2 is a bypass capacitor?

Sort of. It forms a lowpass filter with R3.

C1, C3, and C4 are coupling capacitors?

Of course.

C5 is a filter capacitor?

No. C5 bypasses the emitter resistor R8 of TR3 so that it has max voltage gain.

Then the resistors act as the load resistors, current limiters, and current dividers?

R6 and R7 are a voltage divider, not a current divider.

how would a DPDT switch work to switch the speakers around so that if the left side speaker is the mic and the right-hand speaker is used as the output speaker and then the DPDT switch is thrown and the speakers switch so that now the left-hand side speaker is the output and the right-hand side speaker is the input?

Both speakers have a grounded terminal. A DPDT switch is two switches, one switch for the input of the amplifier and the other switch for the output of the amplifier.

 

[ae4jm]

Audioguru,

Thanks a great deal! I appreciate you working through the schematic with me. I hope to one day be able to look at schematics and imagine the theory behind the circuit and how each component interacts with each other. The numerous configurations that most components can be configured in are an art in itself and I hope to one day learn to understand the techniques used to design such circuits.

Thanks again for all of your time and help!

 

[Hero999]

This circuit isn't very good, the sound will be very distorted and the class A output stage will produce DC in the speaker and use a lot of power.

You'll be much better off with an op-amp preamplifier like the TL071 and a small audio power amplifier like the LM386 to drive the speaker.

 

[mneary]

As suggested by Hero999, you can easily do better. This link **broken link removed** is an example.

 

[ae4jm]

Intercom have 3 amplification stages?

Thanks Hero999 and mneary,

I saw this in one of our electronics labs. I copied the schematic onto paper and thought that I'd try to figure out how the circuit worked. I knew that it was an intercom, the label information was still attached. I appreciate the help, I might decide to build the one from the link to compare the two together; although I'll have to go back and label the values of the initial intercom circuit to rebuild it for a comparison.

Thanks to everyone for all of the great help. I'd really like continue learning about electronics and learn to design, although it looks as if most devices and functionality of anyone's possible wants and demands have been achieved. I suppose there are always new devices, circuits, functions, and there is always the improvement of circuits and functions, much like the one that Hero999 and mneary pointed out.

The initial intercom circuit was labeled as a GE EF-130, from the 70's I think. The circuit was wired up on a breadboard and looked kind of like the learning labs available from Radio Shack, it was on a bread board.

So, I think that I'll continue to read from this forum--the best that I've found to date--and keep reading interesting posts.

Thanks again for the great help and allowing myself the chance to learn more about electronics,
Matt

 

[ae4jm]

So, let me try this again and hopefully I'll be corrected with my misinterpretations, hopefully I won't have many.

The audio is in AC and is coupled to TR1 via C1. VR1 sets the gain of TR1. Then C2 and R3 form a low pass filter which blocks certain frequencies that are amplified through TR1, which can't be estimated right now because there are no known values for R3 and C2, but will more than likely be in the Hz since we are dealing with audio? Do the coupling capacitors block the DC all together and only allow the AC component, which is the audio? Kind of confusing since I thought that AC couldn't be used with transistors?

Next, then audio is coupled from the output of TR1 through the coupling capacitor C3 to TR2, the next amplifier stage for gain. After TR2, C4 couples the output of TR2 to the voltage divider (R6 and R7) where the base of TR3 gets it's negative voltage, which partially turns on TR3 as an output amplifier. Now, R8 is bypassed by C5 for maximum voltage gain for the output of TR3 to the speaker? Why must C4 be bypassed here, DC is blocked again and only audio is allowed, the ac component?

I hope that I haven't made too much of a mess out of attempting to explain and understand the functioning of the circuit and what is actually going on in each stage of the intercom?

Also, why did the PNP have to be used rather than another NPN like the other two transistors in the circuit?

Thanks,
Matt

 

[audioguru]

Do the coupling capacitors block the DC all together and only allow the AC component, which is the audio? Kind of confusing since I thought that AC couldn't be used with transistors?

The coupling capacitors block DC but pass AC audio.
The transistors have DC current. The audio modulates the current so the current increases and decreases at the rate of the audio.

After TR2, C4 couples the output of TR2 to the voltage divider (R6 and R7) where the base of TR3 gets it's negative voltage, which partially turns on TR3 as an output amplifier.

There isn't any negative voltage. The base of Q3 is less positive than its emitter so it conducts a little.

Now, R8 is bypassed by C5 for maximum voltage gain for the output of TR3 to the speaker?

Yes.

Why must C4 be bypassed here, DC is blocked again and only audio is allowed, the ac component?

The collector of Q2 is at a different DC voltage than the base of Q3 so C4 is used to block the different DC voltages but pass the AC audio.

 

[ae4jm]

Thanks for your replies and all of your corrections,

Matt